[156.06] A Long-Lived Accretion Disk Around a Lithium-Depleted Star in Taurus-Auriga

R. J. White, L. A. Hillenbrand (Caltech)

We present a high dispersion optical spectrum of St 34 and
identify the system to be a spectroscopic binary with
components of similar luminosity and temperature (both
M3±0.5). Based on its systemic radial velocity, proper
motion, strong accretion signatures, infrared excess and
location on an H-R diagram, we conclude that St 34 is a
classical T Tauri star and a member of the Taurus-Auriga T
Association. Surprisingly, however, neither component of the
system shows the Li I 6708 A absorption feature in its
spectrum, the most universally accepted criterion for
establishing stellar youth. In this uniquely known instance,
the accretion disk appears to have survived longer than the
lithium depletion timescale. Comparison with pre-main
sequence evolutionary models imply for each component a mass
of 0.37 Msun and an isochronal age of 9 Myr, which is much
younger than the predicted lithium depletion timescale of
~25 Myr. Although a distance closer than that of
Taurus (90 pc versus 140 pc) or a hotter temperature scale
could reconcile this discrepancy, similar discrepancies in
other systems and the implication of an extremely long-lived
accretion disk suggest a possible problem with evolutionary
models. Regardless, with an age of at least ~10 Myr,
St 34 is one of the oldest accreting T Tauri stars known.
The implication is that there may be 10s of non-accreting
Taurus members of early M spectral type that, even if
identified and spectroscopically observed, have been
excluded because they did not pass the lithium test for
youth. If this proposed older population is confirmed, it
would imply that there has been an extended period of star
formation in Taurus. Finally, we speculate based on the high
frequency of spectroscopic binaries among old accreting
systems like St 34 that the presence of a sub-AU separation
companion delays disk dissipation by tidally inhibiting,
though not preventing, circumstellar accretion. Planets may
therefore have a longer time to form in these circumbinary
disks than in circumstellar disks around single stars.